If there's a magic pill for staying youthful, it may be one that's hard to swallow: exercise. Daily doses have been proven to thwart a number of aging factors—stress, obesity, heart disease, diabetes—and the longer you're physically active, the less you'll notice getting older.

The catch is that a 50-year-old's body is not the same as a 20-year-old's; you can't push it the same way you once did, nor should you if you want to keep it in working-out order. So listen to these coaches—they're talking not just professionally but also firsthand—on how to remain fit, and proud of it, through the decades.